What causes cannabis leaves to curl up?

by Nebula Haze

Cannabis leaves curling up? Not a great look! What causes this common marijuana growing problem?

Most common reasons that weed leaves curl up

  • Nutrient deficiency (incorrect pH, nutrients went bad, etc.)
  • Light stress (light too close)
  • Heat stress (too hot)
  • Watering or root problem
  • Cold shock
  • Bugs (especially if there’s a lot of them)

Leaf curling is a sign your cannabis plant is having major problems getting everything it needs to those leaves. Plant processes are not happening normally. There are several potential causes, though it’s often a combination of two or more stressors. For example, strong light combined with a nutrient deficiency can produce insane curling.

When you have cannabis leaf curling, symptoms are often worst close to the light because those are the hardest-working leaves on the plant.

Examples of Leaf Curling and Their Causes

This section is a picture gallery of plants with leaf curling, along with an explanation of what caused the curling.

Bad Nutrients – Can cause all kinds of crazy symptoms.

Sometimes nutrients go bad. If they smell or look different from when you first opened the bottles, they need to be replaced. This particular problem can cause spectacular amounts of leaf curling.

This nutrient deficiency appeared after the grower left their nutrient bottles open, which caused crystals to form (nutrients went bad).

Signs of your nutrients are old and need to be thrown away:

  • Nutrients look different
    • Turned cloudy
    • Crystals forming
    • Changed color
    • Particles floating
  • Nutrients smell different
  • Plant dying and you don’t know why

 

Curling on edges of leaves – light, temperature, or watering problems

When you see the edges of leaves curling up, the cause is often that the light is too close. It can be caused by both light stress and heat. It’s also occasionally caused by cold or watering problems, especially when a plant is cold and overwatered.

Common causes of leaf edges curling up

  • Too much light
  • Heat
  • Cold
  • Humidity (below 35% or above 65% RH, especially combined with poor temperature)
  • Overwatering (especially when combined with cold)
  • Nutrient deficiency

Cold nights plus overwatering

Too much light plus heat.

Edges of leaves are curling up

The air was also dry, which didn’t help.

This marijuana leaf shows the problem. I think this may be humidity problems, but I'm not sure. What do I do?

Heat combined with root problems from chronic overwatering

This cannabis plant is suffering from root problems plus heat

This curling was caused by a broad mites infestation

This was caused by light stress from the grow light too close.

 

Brown splotches plus curling up – Nutrient deficiency, often caused by incorrect pH. Sometimes light or temperature.

Common causes of brown splotches plus curling up

  • Nutrient deficiency
  • Incorrect pH
  • Too much light
  • Heat

Here’s an example of a cannabis plant that has brown splotches on upper leaves due to low pH. The leaves first turn yellow then brown. As they die, the leaves start curling up.

 

These symptoms were caused by high pH at the plant roots. Brown splotches that look like a phosphorus deficiency are common when the pH is too high.

If allowed to progress, the splotches will take over the whole leaf. As the leaves die, they can start curling up.

These brown splotches on the lower leaves of this cannabis plant are the result of a phosphorus deficiency (which was caused by the pH being too low at the roots)

This hydroponic plant experienced too-high pH and suffered a similar fate.

A young cannabis plant with the yellow leaves of a potassium deficiency

These symptoms were caused by heat. This plant was green and healthy but reacted with brown curling leaves after an extreme heat wave.

These twisting curling leaves with brown splotches were caused by heat, overwatering, and constricted roots (plant was in a solo cup)

Pale leaves plus curling – often caused by cold

Cold temperatures (for example if the grow space gets cold at night, or just is cold all the time) can cause some unexpected symptoms, one of which is marijuana leaf curling.

Cold nights turned the new leaves yellow and caused upward curling on the leaf tips.

 

 

Nutrient Burn – Tips of leaves turn brown and eventually start curling up

If you give your plants more nutrients than they can use, it causes nutrient burn. This starts with brown leaf tips, and if not corrected the whole ends will start curling.

This is caused by the plant getting too high levels of nutrients

Read the complete nutrient burn tutorial

Bad grow medium – symptoms similar to overwatering

Here are some last few examples caused by various things.

Leaf curling was caused by bad muddy soil

Pests – can cause a wide variety of symptoms

This curling was caused by a broad mites infestation

Broad mites also cause shiny leaves and twisting. I wanted to include a few extra pictures because broad mites are so tiny they’re difficult to see.

When broad mites attack cannabis they cause droop or twisted new growth, especially near the top of the plant, along with blistered and glossy looking leaves

The following pictures were caused by some mite, though the grower didn’t know if it was hemp russet mites or broad mites

Another view of the same garden

Check out more common cannabis pests if you suspect you may have a bug infestation.

 

Last Thoughts & Hints

Here are a few extra hints if you’re not sure what’s causing your leaf curling

  • If you’ve got a known problem (bugs, heat, nutrient deficiency, etc.) there’s a strong chance it’s at least partly responsible for the curling
  • If you have curling, consider moving your grow light further away until symptoms stop spreading. Even if the light isn’t actually causing the problem, it causes leaves to work more. Moving it further away can help your plant recover faster
  • If cannabis plants are also droopy, it’s likely you’re dealing with watering or root problems
  • Light stress on leaves is common during the second half of the flowering stage because the top leaves stay at the top of the plant for several weeks. For most of your plant’s life, it will be growing new leaves all the time so the top leaves are always relatively young. But by the end of the flowering stage your plant hasn’t grown any new leaves in weeks. The top leaves that survive to this point are probably the hardest working leaves your plant will ever grow.
  • Curled leaves will likely never recover. You know you’ve fixed the problem when you see the symptoms stop spreading.

Want more pictures?

Diagnose my sick cannabis plant!

 


 

GrowWeedEasy.com has hundreds and hundreds of pages!

Check out some articles you may not have seen…

  • Nitrogen Toxicity – Today’s article tells you what to do if your leaves curl downward, but what if they go in the opposite direction? It can actually be a few things, but Nitrogen Toxicity is fairly common since growers of all levels accidentally overfeed their plant sometimes. Check out the page above if your leaves are curling downward and be easy with plant food.
  • 5-Step Solution to 99% of Cannabis Growing Problems – If you have a good method of troubleshooting problems, you’ll solve problems quicker in addition to encountering them less frequently. It’s kind of like having the answers to a test you haven’t even taken yet: you can still mess up with the right answers, but the odds are much more in your favor.
  • How to Make Hash Oil (aka Cannabis Extract Oil or RSO) – Not directly related to this article, but I’m a huge proponent of hash oil.
  • Auto Northern Lights in Grow Box Under 400W HPS – This is a grow journal from way back in the day by a mystery grower. This journal was significant for us because after we posted it, we saw a huge influx of growers building grow boxes in this style. Apparently, a lot of growers can do this kind of stuff and just needed the idea! Let’s see if anyone gets inspired again.

 


 

 

 

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